"Montreal was the best record broken, hitting 35.2 C, breaking a daily record set back in 1955," Matthews said.

Montreal public health officials were reminding people that many are at a serious risk because of the heat. In May, the public health agency published a report stating that in 2010, 106 deaths in the city were likely caused by heat waves.

Agency officials said the elderly are the most vulnerable, but this year they're also reaching out to those suffering from mental health issues.

Cities set up cooling centres

The heat sent residents of several Ontario cities to emergency cooling centres, slowed commuter trains and prompted the Blue Jays to close the retractable roof of Toronto's Rogers Centre for their afternoon game against the Mariners (the first time it has been closed because of the heat).

Due to the extreme heat in Toronto, Air Canada Cargo imposed a temporary embargo on transporting animals via cargo for animal health and safety reasons. The embargo is reviewed regularly and has been extended for another 24 hours due to forecast temperatures, Air Canada spokeswoman Angela Mah told CBC News at 5 p.m. ET Thursday.

Thursday afternoon's thoroughbred racing card at Woodbine was cancelled because of the heat. The decision was made after the racetrack's management consulted with the Horsemen's Benevolent and Protective Association and the Ontario Racing Commission veterinarians.

Guide to the humidex

The humidex is an index number without units that measures the combined effect of heat and humidity. The humidex number is equivalent to the dry temperature in degrees Celsius that it feels like to someone outside in the heat.

What it feels like when the humidex is:

30 to 39: Some discomfort.

40 to 45: Great discomfort; avoid exertion.

Above 45: Dangerous.

Above 54: Heat stroke imminent.

"In the best interest of the horses and after having discussed it with the HBPA and the veterinarians, it's the right decision to cancel Thursday's thoroughbred card," said Steve Koch, WEG's vice-president of thoroughbred racing. Thoroughbred racing is expected to resume at Woodbine on Friday. at 2 p.m.

Canada Post mail carriers in heat wave regions were allowed to start their routes in the early-morning hours, so they're not out during the hottest part of the day, said spokesman John Caines. Carriers were also given sunscreen, hats and water.

"They're professionals as well, so they know how to conduct themselves in this kind of weather, but if they can get out early and get the mail before it gets too, too hot and get it all delivered, then it's better for everybody," he said.

Despite the soaring temperatures, the province's power supply was expected to meet demand. The Independent Electricity System Operator predicts peak demand will hit 25,591 megawatts today, which won't even be enough to crack the top 10 peak demand days.

The all-time high was Aug. 1, 2006, when Ontario needed 27,005 megawatts of electricity. IESO spokesman Terry Young said lower industrial demand is one of the reasons we won't set any power consumption records despite the soaring heat and humidity.

The heat also forced GO Transit commuter trains to travel slower than usual as a safety precaution because the metal tracks they ride on expand in the heat. The transit operator was warning that it could mean 10- to 15-minute delays.

The rising temperatures have been caused by a so-called heat dome — a hot, unmoving high-pressure area hovering over central Canada.

The dome is pushing the jet stream well to the north and keeping cooler or wetter weather out. The same phenomenon has also caused a heat wave in the U.S., the worst in more than a decade.